Seismic isolation involves isolating structures and damping the isolated structures from incoming forces due to earthquakes. New Zealand Patent 178949 describes a very successful bearing which may be used to isolate structures in this way. The favoured embodiment of this invention is referred to as a lead rubber bearing. A more comprehensive discussion of seismic isolation and the lead rubber bearing can be found in "An Introduction to Seismic Isolation", Skinner et, John Wylie and Sons (1993).
Another means of seismically isolating structures is a bearing sold under the trade mark "Frictional Pendulum" by Earthquake Protection Systems Inc, of Alameda, Calif., USA. Such a bearing consists of a pendulum member with a frictional surface at the distal end of the pendulum member which slides over a polished surface. This is described in U.S. Pat. No. 4,644,714.
Another device intended to provide seismic isolation is described in the U.S. Pat. No. 4,726,161. This apparatus includes a rollable bearing and a bowl. A spring urges the rollable bearing into contact with the bowl. The combination dampens seismic forces on a structure supported on said rollable bearing.
In U.S. Pat. No. 5,599,106 there is described a seismic isolating bearing. This consists in upper and lower load plates with downward and upward facing rigid surfaces, respectively, at least one of which is conical. A rigid ball is sandwiched between the plates. The ball rolls up a conical surface when one load plate is displaced laterally from the other by an external force, and returns to the centre of the core by gravitational forces when the external force ceases.
The invention described in WO97/48866 is a seismic isolator consisting of a ball interposed between a foundation and a building. The ball is retained in a cage like structure secured to the underside of the building. It rolls on a hemispherical surface secured to the foundation.
In all of the prior art devices described above the bearing element, or ball, and the surfaces on which it rolls are substantially incompressible. This means that when there is relative movement between the structure being supported and the surface upon which the bearing rests there is no braking force being applied apart from the weight of the structure itself. If the incoming forces causing relative movement are long lasting then it can take a considerable amount of time for the structure to come to rest.
It is an object of this invention to go some way towards overcoming this disadvantage or at least to offer the public a useful choice.